<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            padding: 0;
            margin: 0;
        }
        
        .wrap {
            width: 300px;
            margin: 100px auto 0;
        }
        
        table {
            border-collapse: collapse;
            border-spacing: 0;
            border: 1px solid #c0c0c0;
            width: 300px;
        }
        
        th,
        td {
            border: 1px solid #d0d0d0;
            color: #404060;
            padding: 10px;
        }
        
        th {
            background-color: #09c;
            font: bold 16px "微软雅黑";
            color: #fff;
        }
        
        td {
            font: 14px "微软雅黑";
        }
        
        tbody tr {
            background-color: #f0f0f0;
        }
        
        tbody tr:hover {
            cursor: pointer;
            background-color: #fafafa;
        }
    </style>

</head>

<body>
    <div class="wrap">
        <table>
            <thead>
                <tr>
                    <th>
                        <input type="checkbox" id="j_cbAll" />
                    </th>
                    <th>商品</th>
                    <th>价钱</th>
                </tr>
            </thead>
            <tbody id="j_tb">
                <tr>
                    <td>
                        <input type="checkbox" />
                    </td>
                    <td>iPhone8</td>
                    <td>8000</td>
                </tr>
                <tr>
                    <td>
                        <input type="checkbox" />
                    </td>
                    <td>iPad Pro</td>
                    <td>5000</td>
                </tr>
                <tr>
                    <td>
                        <input type="checkbox" />
                    </td>
                    <td>iPad Air</td>
                    <td>2000</td>
                </tr>
                <tr>
                    <td>
                        <input type="checkbox" />
                    </td>
                    <td>Apple Watch</td>
                    <td>2000</td>
                </tr>

            </tbody>
        </table>
    </div>
</head>
<body>
    <script>
     //1. 事件源 
    var  qq=document.querySelector('#j_cbAll')
         var wx = document.querySelectorAll('#j_tb input')
         //2.事件类型
     qq.onclick = function(){
//3.事件程序
         for(var i=0;i<wx.length;i++){
             wx[i].checked = this.checked
         }
     };  
     //反选  
     //  获取所有复选框遍历依次绑定事件
     //for (var i = 0; i < wx.length; i++){
      //var wx = document.querySelectorAll('#j_tb input')

          for (var i = 0;i < wx.length; i++){
         wx[i].onclick =function(){
             //依次询问你们是否都被选中了
            //  如果所有复选框都选中了 那么我们应该让qq中的复选框选中
            //假设一个变量全部选中
             var ft=true
             for(var i = 0;i<wx.length;i++){
                //  console.log(wx[j].checked);
                 if(wx[i].checked === false){
                     ft=false
                     break
                 }
             }
             qq.checked = ft
         }
     }  

    </script>
</body>
</html>